In a passive matrix LCD display device, the lines and columns of the display cell are controlled by a conventional control or drive circuit. The number of lines is generally less than the number of columns. The pixels of each column are addressed one after the other by line-by-line multiplexing. The pixel is off between two refreshing operations. Normally, when the dimensions of the display device are large, an active matrix LCD display device is used.
For a display device with a display cell of large dimensions, one could envisage configuring the display device so that it uses only an active part of the cell in partial display mode, in order to reduce the electrical consumption of the device in the partial mode. EP Patent Application No. 0 811 866, which discloses a method of driving a display device so as to reduce the electric power consumption thereof, can be cited in this regard. To achieve this, part of the inactive lines and columns of the matrix are placed at a defined voltage. Lines and columns of the active part are, however, addressed in succession in a conventional manner, particularly by multiplexing, for the display of data.
One drawback of this solution disclosed in EP Patent Application 0 811 866 is that, in complete display mode or in partial display mode, several voltage levels must be used to display data. These different voltage levels have to be obtained by at least one voltage booster circuit. This means that the electric power consumption cannot be sufficiently reduced even in partial display mode, since line-by-line multiplexing at different voltage levels has to be performed.
Instead of multiplexing the lines, an addressing method was disclosed in the article entitled “Active Addressing Method for High-Contrast Video-Rate STN Displays”, in SID 1992, by Messrs T. J. Scheffer and B. Clifton. The object of the method disclosed is to overcome problems linked to LCD display devices, which are always quicker. The principle is to apply orthonormed functions to the lines and calculate the values on the columns as a function of the values to be displayed by the pixels and the values on the lines. However, this article does not disclose the means of making a display device able to operate at low power as for the present invention.
EP Patent Application 0 617 397 discloses a liquid crystal display device of large dimensions. Several line drive circuits drive several lines of this display device individually and simultaneously with voltages determined in accordance with Walsh functions. However, nothing is specified regarding a reduction in power consumption upon passage from complete display mode to partial display mode, in which an active addressing technique can be used.